Last night while playing multiplayer, I would sell some extra food or water.. My money would go from 5000 to 200 or so. This happened on and off for 5 minutes when I just asked everyone to restart the game. We did and it was fixed. Then, later it happened to another player.

Most likely what's happening is there is a building you own but aren't producing the resources to keep online. In this case the "auto-supply" option will have the building automatically purchase the resources it needs. This option can be enabled/disabled by building type in game and as a player option out of game.